Eaves was circling the Pittsburgh net midway through the second period when he took a stiff check to the head from Penguins forward Colby Armstrong, knocking him hard to the ice. While Eaves lay prone, Senators forward Dean McAmmond fought with Penguins forward Maxime Talbot.
Eaves was alert, and was shown speaking to doctors and flexing his hands as he was wheeled off the ice. One media source reported Eaves had movement in all his extremities, and was diagnosed with a concussion.
His status for Game 4 on Tuesday was not certain, but a concussion would likely keep him out.
Armstrong wasn't given a penalty on the play. The Penguins forward was involved in two similar plays this season. On Oct. 14, Carolina forward Trevor Letowski was knocked unconscious and sustained a concussion when Armstrong put a blind-side hit on him in the Hurricanes' end.
Letowski, who also sustained facial cuts, missed nine games because of the concussion.
On Feb. 1, Armstrong put a hard hit on the Canadiens' Saku Koivu near the end boards but also wasn't penalized, though Montreal's Sheldon Souray immediately went after Armstrong. A day later, Canadiens coach Guy Carbonneau labeled Armstrong's hit "a cheap shot."